The present invention relates to explosion mitigation for access openings, and more particularly to a vault panel configured to release pressure during an explosion event.
Vaults generally are enclosures that may be located fully or partially below grade in or near a road, a walkway, or in a basement of a residential or commercial building. Such vaults may house electrical or other equipment that occasionally smoke or may catch fire and could cause an explosion within the vault. There are many examples of combustible gases to be found in subsurface structures that contain electrical equipment. Cable insulations are typically hydrocarbon based materials such as cross-linked polyethylene (XLPE), ethylene propylene rubber (EPR), polyethylene (PE) and polyvinyl chloride (PVC) that are flammable if they are vaporized and exist within their respective flammable limits. Transformers, voltage regulators and oil-filled switches that are often installed in vaults utilize dielectric fluids that are combustible when vaporized as a result of a damaging fault. Insulating oils employed in paper insulated lead covered (PILC) cables and in self-contained fluid-filled (SCFF) cables are combustible and can add chemical energy to an explosion event if they ignite during a failure.
Aging cables, splices, and joints that may have been overloaded in the past can emit combustible gases as insulation materials overheat and degrade. If these gases are emitted in sufficient quantities such that they reach their flammable limits, then gas explosions can occur in the presence of an ignition source. Other likely sources of combustible gases can be traced to salt or other chemicals that are applied to streets during winter months; gases present due to the public disposal of combustible materials in the streets; decaying organic material that finds its way into underground structures; and gases from leaks in pipelines.
Somewhat limited access to a vault may be by a removable vault panel. For example, one wall of the vault—generally the top wall—may have an opening for receiving a vault panel. The vault panel can be a removable panel over the access opening providing ingress/egress for the underground area. The panel may, among other things, prevent a person from falling in and can restrict entry of unauthorized persons. Such panels may be seated on a frame installed over the access opening and may be flush with an at-grade surface such as a road, walkway, or basement floor. The frame may provide an interface for the panel to seat over the opening within the vault wall. Generally, the panel is secured to the frame or surrounding structure so that the panel may be removed for access while the frame remains fixed in place.
The vault panels often weigh more than 100 pounds, partly because the weight keeps them in place when traffic passes over them, and partly because they are often made out of concrete, sometimes with steel or cast iron frames. This makes them inexpensive and strong, but heavy. Despite their substantial weight, explosions in the area underneath the panel may unseat the panel due to the sudden pressure build up from the explosion. In many cases, the explosion events may move the panel so that the access opening is left fully or partially uncovered. And, in some explosion events, the panel may become a projectile, flying up to 20 feet or more in the air. To address these potentially undesirable effects, vault panels are sometimes bolted or fastened to the frame. This configuration, however, may not fully or quickly relieve the pressure from an explosion event, thereby potentially causing structural damage to the underground area or to surrounding surface structure, such as the road or building.
In most cases where vaults are installed, the vault panels include a smaller opening within the panel that receives a manhole cover. The manhole cover may be the primary access point for the inside of the vault, with the vault panel forming a secondary access method for larger, heavier infrastructure that needs to be placed within the vault. These manhole covers are generally standard and well known; however, some manhole cover assemblies have been developed to release explosive pressure in a controlled fashion. These assemblies behave similar to a relief valve by allowing the manhole cover to rise in response to explosive pressure and to then reseat once the pressure has subsided. In this way, the manhole cover may be prevented from becoming a projectile, or from leaving the frame or access opening uncovered, in response to an explosive event.